1. Field of the Invention
This invention relates to a thermoelectric converter element consisting of substance capable of thermoelectric conversion (to be referred to as "thermoelectric substance" hereinafter) which produces an electromotive force across opposite ends thereof upon application of a temperature difference across said opposite ends. More particularly, the invention improves the performance of the thermoelectric converter element of the above-mentioned type by controlling the crystal structure thereof.
2. Description of the Prior Art
An example of thermoelectric converter elements is the so-called thermo-couple consisting of a pair of metallic members of different kinds which are joined at one ends thereof, so that an electromotive force is generated across the joined ends of the metallic members and the opposite ends thereof upon application of a temperature difference across said joined ends and said opposite ends. A thermoelectric converter element made of semiconductor materials and having a similar structure as that of the conventional thermo-couple is also known. For instance, a semiconductor thermoelectric converter element using the Seebeck effect is formed by connecting one end of a p-type semiconductor member of bar shape to one end of a similarly shaped n-type semiconductor member through a metallic plate and connecting separate electrode plates to the opposite ends of the two semiconductor members as output terminals of the converter element. The joined ends of the two semiconductor members may be kept at a high temperature Th together with the metallic plate, while the opposite ends thereof may be kept at a low temperature Tc together with the electrode plates, so that a temperature difference is applied across the opposite ends of the p-type and n-type semiconductor members. Whereby, a positive potential V.sub.+ may be produced at the open end of the p-type semiconductor member, while a negative potential V.sub.- may be produced at the open end of the n-type semiconductor member. The phenomenon of producing an electromotive force across the joined ends of two semiconductor members of different kinds and the opposite ends thereof is well known as the Seebeck effect and widely used in the thermoelectric converter elements for converting thermal energy into electric energy.
In the thermoelectric converter elements of the above-mentioned type, the p-type and n-type semiconductor members of bar shape are made of substance with block-state structure (to be referred to as block-state substance, hereinafter) formed by a melting method, Bridgman method, or a powder sintering method. The block-state substance made by such methods has polycrystal structure in the main, and it has been impossible to control the crystallographic axis thereof in a specific orientation at will with a high reproducibility. Accordingly, it has been impossible to evaluate the crystal structure of the thermoelectric substance in the case of the conventional thermoelectric converter elements made of semiconducting block-state substance and using the Seebeck effect. In fact, no attention has been paid to the crystal structure or crystallography of such semiconducting block-state substance.
On the other hand, from the standpoint of thermoelectric conversion efficiency of such thermoelectric converter elements, it is necessary to improve the performance index Z of the thermoelectric substance, which performance index Z is given by EQU Z=.alpha..sup.2 .sigma./.kappa.
here,
.alpha.: thermoelectric power, namely, the rate of change of the induced voltage E with respect to the variation of temperature T, i.e., dE/dT, PA1 .sigma.: electric conductivity, and PA1 .kappa.: heat conductivity.
Thus, it is desirable to form a thermoelectric converter element by using a substance which has a large thermoelectric power .alpha., a large electric conductivity .sigma., and a small heat conductivity .kappa.. However, in the conventional thermoelectric converter elements of the above-mentioned type, it has been impossible to arbitrarily control the crystal structure of the thermoelectric substance as pointed out above, so that it has been impossible to improve the above-mentioned performance index Z of such thermoelectric substance. In fact, no attention has been paid to the improvement from the standpoint of crystal structure. Thus, the prior art has a shortcoming in that major efforts for improving the performance of the thermoelectric substance have been only in the searching of substances with a large thermoelectric power .alpha. which is considered to be one of the inherent properties of specific substance.
Besides, the conventional thermoelectric converter elements have a shortcoming in that the thermoelectric substance made by the conventional methods are always in the block state, so that it has been very difficult to make such thermoelectric substance in miniature size or thin film, and it has been impossible to form the thermoelectric substance into integrated circuits with a high output voltage and a large output current.